This technology relates to optical switching devices incorporating tunable filters and bandblock reflectors. The tunable filters include devices (typically semiconductor devices) that produce quantum effects. For the purposes of this document, the term “optical” refers to visible, ultraviolet (UV), and infrared (IR) light which obey the normal rules of optics. By this definition, long-wavelength infrared, microwaves, radio waves, extreme ultraviolet, x-ray, and gamma radiation are not optical radiation. Optical filters and switches block light by absorbing or reflecting certain frequencies while allowing others to pass through. Short-pass and long-pass filters (specific to wavelength) or high-pass and low-pass filters (specific to frequency) may be used, or a narrow range of wavelengths/frequencies can be blocked by a notch filter or bandblock filter, or transmitted by a bandpass filter.
Semiconductors are capable of serving as filters in several ways. The optical response of a semiconductor is a function of its bandgap—a material-specific quantity. For photons with energies below the bandgap, the semiconductor is generally transparent, although material-specific absorption bands may also exist. Photons with energies higher than the bandgap are capable of creating electron-hole pairs within the semiconductor, and are therefore generally absorbed or reflected. For example, a material like gallium arsenide (GaAs) (bandgap ˜1.424 eV) is transparent to infrared photons with a wavelength of 871 nm or greater, and opaque to visible light, whereas silicon dioxide (SiO2) (bandgap ˜9.0 eV) is transparent to visible and near-ultraviolet light with a wavelength greater than 138 nm. Thus, semiconductor materials are capable of serving as optical, infrared, or ultraviolet longpass filters.
A semiconductor material will also generally show a strong emission or luminescence peak at this bandgap energy or cutoff energy, i.e., when stimulated with an electrical current, or with absorbed photons of higher energy, the semiconductor material will emit photons at the cutoff energy as a result of electron-hole recombinations within the material. Photoluminescence (i.e., stimulating the material with high-frequency light and measuring the resulting fluorescence or emission spectrum) is therefore useful as a diagnostic tool to determine the quantum confinement energy of a quantum well and thus predict its optical properties. Strong absorption at and above the cutoff energy is also capable of generating photoelectric effects within the semiconductor as large numbers of electron-hole pairs are created.
The fabrication of very small structures to exploit the quantum mechanical behavior of charge carriers, e.g., electrons or electron “holes” is well established. Quantum confinement of a carrier can be accomplished by a structure whose dimension is less than the quantum mechanical wavelength of the carrier. Confinement in a single dimension produces a “quantum well,” and confinement in two dimensions produces a “quantum wire.” A “quantum dot” is a structure capable of confining carriers in all three dimensions. Some filters also incorporate quantum wells, quantum wires, or quantum dot particles as dopants (much as leaded crystal incorporates lead atoms or particles as dopants) to affect the behavior of the filter. However, the optical properties of such filters are fixed at the time of manufacture and are neither multifunctional nor programmable.
The energy of an electron confined in a quantum well is not only a function of bandgap, but of the quantum confinement energy, which depends on the thickness of the well and the energy height of the surrounding barriers (i.e., the difference in conduction band energy between the well and barrier materials). This “bandgap plus quantum confinement” energy moves the transparency of the material into shorter wavelengths. Thus, while a bulk GaAs sample emits and absorbs photons at approximately 870 nm, a 10 nm GaAs quantum well surrounded by Al0.4Ga0.6As barriers has a 34 meV quantum confinement energy and thus shows the equivalent cutoff at approximately 850 nm. Therefore, for a given set of materials and a given reference temperature, the cutoff energy can be fixed precisely through the fabrication of a quantum well of known thickness. It should be noted, however, that the bandgap is a temperature-dependent quantity. As the temperature of a semiconductor decreases, its bandgap increases slightly. When the semiconductor is heated, the bandgap decreases.
Quantum dots can be formed as particles, with a dimension in all three directions of less than the de Broglie wavelength of a charge carrier. Quantum confinement effects may also be observed in particles of dimensions less than the electron-hole Bohr diameter, the carrier inelastic mean free path, and the ionization diameter, i.e., the diameter at which the quantum confinement energy of the charge carrier is equal to its thermal-kinetic energy. It is postulated that the strongest confinement may be observed when all of these criteria are met simultaneously. Such particles may be composed of semiconductor materials (for example, Si, GaAs, AlGaAs, InGaAs, InAlAs, InAs, and other materials) or of metals, and may or may not possess an insulative coating. Such particles are referred to in this document as “quantum dot particles.”
A quantum dot can also be formed inside a semiconductor substrate through electrostatic confinement of the charge carriers. This is accomplished through the use of microelectronic devices of various designs, e.g., an enclosed or nearly enclosed gate electrode formed on top of a quantum well. Here, the term “micro” means “very small” and usually expresses a dimension of or less than the order of microns (thousandths of a millimeter). The term “quantum dot device” refers to any apparatus capable of generating a quantum dot in this manner. The generic term “quantum dot” (abbreviated “QD” in certain of the drawings herein) refers to the confinement region of any quantum dot particle or quantum dot device.
The electrical, optical, thermal, magnetic, mechanical, and chemical properties of a material depend on the structure and excitation level of the electron clouds surrounding its atoms and molecules. Doping is the process of embedding precise quantities of carefully selected impurities in a material in order to alter the electronic structure of the surrounding atoms for example, by donating or borrowing electrons from them, and therefore altering the electrical, optical, thermal, magnetic, mechanical, or chemical properties of the material. Impurity levels as low as one dopant atom per billion atoms of substrate can produce measurable deviations from the expected behavior of a pure crystal, and deliberate doping to levels as low as one dopant atom per million atoms of substrate are commonplace in the semiconductor industry, for example, to alter the conductivity of a semiconductor.
Quantum dots can have a greatly modified electronic structure from the corresponding bulk material, and therefore different properties. Quantum dots can also serve as dopants inside other materials. Because of their unique properties, quantum dots are used in a variety of electronic, optical, and electro-optical devices. Quantum dots are currently used as near-monochromatic fluorescent light sources, laser light sources, light detectors including infra-red detectors, and highly miniaturized transistors, including single-electron transistors. They can also serve as a useful laboratory for exploring the quantum mechanical behavior of confined carriers. Many researchers are exploring the use of quantum dots in artificial materials, and as dopants to affect the optical and electrical properties of semiconductor materials.
The embedding of metal and semiconductor nanoparticles inside bulk materials (e.g., the lead particles in leaded crystal) has occurred for centuries. However, an understanding of the physics of these materials has only been achieved comparatively recently. These nanoparticles are quantum dots with characteristics determined by their size and composition. These nanoparticles serve as dopants for the material in which they are embedded to alter selected optical or electrical properties. The “artificial atoms” represented by these quantum dots have properties which differ in useful ways from those of natural atoms. However, it must be noted that the doping characteristics of these quantum dots are fixed at the time of manufacture and cannot be adjusted thereafter.
Tunable filters rely on various mechanical principles such as the piezoelectric squashing of a crystal or the rotation or deformation of a lens, prism, or mirror, in order to affect the filter's optical properties. Most notable of these is the Fabry-Perot interferometer, also known as an “etalon.” Like any mechanical device, such tunable filters are much more vulnerable to shock, vibration, and other related failure modes than any comparable solid-state device.
The addition of a mechanical shutter can turn an otherwise transparent material—including a filter—into an optical switch. When the shutter is open, light passes through easily. When the shutter is closed, no light passes. If the mechanical shutter is replaced with an electrodarkening material such as a liquid crystal, then the switch is “nearly solid state”, with no moving parts except photons, electrons, and the liquid crystal molecules themselves. This principle is used, for example, in liquid crystal displays (LCDs), where the white light from a backdrop is passed through colored filters and then selectively passed through or blocked by liquid crystal materials controlled by a transistor. The result is a two-dimensional array of colored lights which form the pixels of a television or computer display.
A single-electron transistor (SET) is a type of switch that relies on quantum confinement. The SET comprises a source (input) path leading to a quantum dot particle or quantum dot device, and a drain (output) path exiting, with a gate electrode controlling the dot. With the passage of one electron through the gate path into the device, the switch converts from a conducting or closed state to a nonconducting or open state, or vice-versa. However, these devices are not designed to control the flow of optical energy (i.e., light).
Band reflectors may be constructed by a variety of different means. In general, a band reflector is a filter that consists of transparent materials of different indices of refraction or different dielectric constants, such that certain frequencies or frequency bands of light are strongly interfered with (reflected) while other frequencies pass through with minimal reflection or attenuation. Thus, a band reflector is highly transparent across a broad range of frequencies, and highly reflective within a narrow band of frequencies. Band reflectors are used, for example, as cavity mirrors in certain types of lasers.
Each of these optical filters, switches, and combinations described above are not programmable or multifunctional. That is, they always pass or block the exact same wavelengths/frequencies of light, which are determined at the time of manufacture and cannot be altered thereafter.
Thermochromic materials change their color (i.e., their absorption and reflection spectrum) in response to temperature. Liquid crystal thermometers and liquid crystal tunable filters (LCTFs) are based on this principle. Thermochromic plastics are sometimes incorporated into baby bathtubs, bottles, or drinking cups as a visual indicator of liquids that may be too hot or too cold for safety or comfort. Thermochromic paints are sometimes used to help regulate the temperature of objects or buildings under heavy sunlight.
The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.